1. Field of Invention
This invention relates to a low-temperature method of depositing magnetic iron oxide films, ferrites, and, more particularly, to a method of formation of magnetite (Fe.sub.3 O.sub.4) and .gamma.Fe.sub.2 O.sub.3 films on a substrate which is not a single crystal. The results produced are useful as magnetic recording media and magnetic recording head layers.
2. Description of the Prior Art
Thin film magnetite films have been specifically described in U.S. Pat. No. 3,860,450 of Nicolet et al., for a "Method of Forming Magnetite Thin Film," in which a thin film of iron is deposited onto a substrate by vacuum deposition, decomposition of iron carbonyl or R.F. sputtering onto a substrate from an iron target. Then the iron is oxidized by heating at 450.degree.-550.degree. C in the presence of oxygen and more iron is deposited upon the resultant iron oxide, which comprises principally hematite (.alpha.Fe.sub.2 O.sub.3). Then the resultant films are annealed preferably in a vacuum at 350.degree. to 400.degree. C to yield a green magnetite (Fe.sub.3 O.sub.4) film. Then the excess iron is stripped away from the underlying magnetite film by means such as dipping the coated substrate in a nitric acid solution.
The above film possesses desirable magnetic characteristics, but is unsuitable for use as a high-density magnetic recording medium because of the roughness of the resultant film, with peaks-to-valleys on the order of or greater than 1000A (0.1 micron). The roughness is caused by the step of thermal oxidation.
Even if the above film were sufficiently smooth, it would be unsuitable for use with flexible magnetic recording substrates such as flexible discs and tapes because of the high temperatures of 450.degree. and 350.degree. C required for the two steps involved which would totally destroy most flexible media substrates.
A question may be raised as to why .alpha.Fe.sub.2 O.sub.3 and Fe are undesirable in such thin films. Coupled with that question is a further question as to why iron oxide, ferrites, and particularly Fe.sub.3 O.sub.4 cannot be sputtered successfully by conventional techniques onto amorphous substrates to yield high quality magnetic films. These questions are answered by the fact that while ferrites, .gamma.Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4 possess desirable magnetic properties, .alpha.Fe.sub.2 O.sub.3 and Fe do not, and even small quantities of them in a structure containing .gamma.Fe.sub.2 O.sub.3 and/or Fe.sub.3 O.sub.4 hurt the magnetic properties of the thin film. Pure Fe in the films is undesirable because it would make the films susceptible to corrosion. Further, when an iron oxide is sputtered onto an amorphous substrate without epitaxial constraint, substantial amounts of Fe and/or .alpha.Fe.sub.2 O.sub.3 are formed, leading to unacceptably poor magnetic properties. See U.S. Pat. Nos. 3,342,632 of Bate et al., 3,342,633 of Bate et al., 2,853,401 of Rogers, and 3,829,372 of Heller.
In an article by H. Takei et al., "Vacancy Ordering in Epitaxially Grown Single Crystals of .gamma.Fe.sub.2 O.sub.3," Journal of the Physical Society of Japan, Vol. 21, p. 1255 (1966), epitaxial chemical vapor deposition of Fe.sub.2 O.sub.3 at 600.degree.-700.degree. C on a single crystal MgO substrate allows synthesis of single crystal .gamma.Fe.sub.2 O.sub.3 films. This approach is adequate only where the great cost and expense of providing a single crystal MgO substrate can be justified, which is not usually the case. See U.S. Pat. No. 3,498,836 of Gambino related to epitaxial deposition of ferrites on an MgO single crystal, but at temperatures in the range of 1050.degree. - 1300.degree. C.
U.S. Pat. No. 3,520,664 of York discloses a thin film structure with a substrate of a metal or a dielectric such as glass coated with a first film of an adhesive metal such as Cr, Ta, Nb, or Mo. Next, is an insulating layer such as SiO. The next layer is an electrically discontinuous nucleating layer such as Ag, Cr, Co, Ta, Fe, Au, Ni, V, and Ti. The final layer is an Ni, Fe, or an Ni, Mo, Fe form of permalloy. The nucleating layer is intended to provide "nucleating centers around which a subsequent magnetic film may grow. Thus, the layer of nucleating material serves to form small agglomerations, evenly dispersed over the surface of the insulating layer."
The nucleating layer is not intended to provide an epitaxial influence on the subsequent magnetic layer, but it is intended to precondition the substrate surface to favor the formation of a better defined magnetic film. These nucleating layers play no role in controlling the stoichiometry of the permalloy deposited on the film. Such discontinuous layers would prevent formation of uniform and stoichiometric ferrite films, particularly .gamma.Fe.sub.3 O.sub.3 and Fe.sub.3 O.sub.4. Silver is face centered cubic, but has improper lattice parameters. Titanium has a hexagonal crystal structure which is the wrong crystal structure. Tantalum has a body-centered cubic structure, but has dimensions of 9.33A by 9.90A which is inappropriate. See Table II below and the further discussion in connection with it.
U.S. Pat. No. 3,515,606 shows a layer of 300A of chromium on a glass substrate covered with 1500A of NiFe where the chromium is added to increase adhesion.
U.S. Pat. No. 3,516,860 shows a layer of chromium deposited on a glass disc with a layer of CoAg recording medium deposited on the chromium.
U.S. Pat. No. 3,677,843 of Reiss also describes permalloy layers on chromium.
U.S. Pat. No. 3,441,429 of Hacskaylo describes vacuum deposition of Fe.sub.3 O.sub.4 mixed with B.sub.2 O.sub.3.
U.S. Pat. No. 3,787,237 of Grunberg et al describes alternate layers of Cr, Co, Cr, Co with the Cr layers as thin as possible to form a thin film with a high coercive field.